High-speed image processing equipment has made possible the processing of image data to present new scenes or perspectives of an object from image data of a different scene or perspective. For example, digital elevation information of a geographical region can be processed to generate a variety of different two and one-half dimensional representations of the terrain. The new perspective scenes of the terrain can be generated by rendering techniques to give the observer a perspective view of the terrain from any point in space. From a series of such perspective views of the subject terrain, a person can experience a hypothetical flight over the terrain without actually experiencing a true flight. It can be appreciated that such a rendering technique is highly advantageous in military and aeronautical applications.
Image data related to three-dimensional objects may also be rendered to further define, clarify, create, etc., new images which in reality are difficult or impossible to observe. Such applications include physiological cat-scans, X-rays, sonograms, nuclear magnetic resonance, etc. Given such three-dimensional image data, perspective views can be generated using translucency, opacity or color parameters to create new perspective views to accentuate body tissues or other material.
Image rendering can be achieved by various techniques, including image ray tracing. Ray tracing techniques are disclosed in connection with volume rendering in the technical article entitled, "Display of Surfaces From Volume Data", by Marc Levoy, IEEE, Computer Graphics and Applications, May, 1988 pages 29-37. According to such techniques, and other similar ray tracing techniques, point is established in free space from which hypothetical rays are extended toward a reflective image of the object. In two and one-half dimensional ray tracing, the image processor proceeds in successive increments along each ray to determine if the surface of the object has been penetrated. Each ray increment or section corresponds to a pixel location associated with the reflective image. When the surface of the image is penetrated by a particular ray section, the pixel information or resampled information thereof, is stored for later presentation of the new perspective view.
According to three-dimensional image ray tracing, each ray is also incrementally traversed, noting whether any point of interest is within a specified volumetric vicinity. Regions of interest may include arbitrary intensity ranges or opacity ranges existing within a given sphere in which the ray section is centered.
One shortcoming of conventional image ray tracing is the time consumed in performing the rendering operation. The increments by which each ray is traced are generally small. Because information for a large number of pixels must be processed at each point along the ray, the processing time of all the rays can become enormous. For example, in rendering a typical terrain object in two and one-half dimensions, the rendering can take ten to fifteen minutes depending upon the complexity of the base reflective image. Because a three-dimensional rendering involves cubic dimensions, rendering of associated images may take even longer. Hence, it can be seen that a need exists for an improved technique which reduces the time to carry out a rendering operation. A further need exists for a new rendering operation which is faster than current techniques, but which does not compromise the quality of the resultant perspective view.